I'm surprised the Impala rating is only 1000 pounds. What size engine does it have? As cars go these days, the Impala is a decent size vehicle. I would have expected it to be 2000 pounds.
While it is never good to exceed mfg ratings, there is a 'fudge factor' built in, knowing that humans being humans will push the limit. If your rig grossed out at 1200 pounds it probably would be safe for the occasional fishing trip. If you plan to tow long distances frequently, might need to consider a different car.